6.1.3.5
Filtering
to
reduce
measurement
noise
An
adjustable
filter
is
applied
to
analog
ue
measurements,
reducing
signal
components
at
selected
frequencies.
The
following
figure
shows
the
filter
frequency
response.
Using
the
first
notch
frequency
(
fN1
)
parameter,
users
can
select
the
placement
of
the
filter
notches.
The
first
notch
falls
at
the
specified
fN1
,
and
subsequent
notches
fall
at
integer
multiples
of
fN1
.
Commonly,
fN1
is
set
at
50
or
60
Hz
to
filter
50
or
60
Hz
signal
components,
reducing
noise
from
ac
power
mains.
Filtering comes at the expense of measurement time. The time required for filtering is equal to
1/f
N1
. For example, setting
fN1
equal to 50 will require 1/50 sec (20 ms) for filtering. As
fN1
is
set to smaller values, random noise in the measurement results decreases, while measurement
time increases. The total time required for a single result includes se fil overhead.
Consult the following technical paper at
for in-depth treatment
of measurement noise:
Preventing and Attacking Measurement Noise Problems
.
CR1000X filtering details
The datalogger utilizes a sigma-delta ADC that outputs digitized data at a rate of 31250 samples
per second. User-specified filtering is achieved by averaging several samples from the ADC.
Recall that averaging the signal over a period of 1/f
N1
seconds will filter signal components at f
N1
Hz. The final result, then, is the average calculated from 31250/f
N1
samples. For example, if
fN1
is set to 50 Hz, 625 samples (31250 / 50) are averaged to generate the final filtered result.
The actual f
N1
may deviate from the user-specified setting since a whole integer number of
samples must be averaged. For example, if
fN1
is set to 60 Hz, 521 samples (31250 / 60 = 520.83)
6. Measurements
53
Содержание CR1000X
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